1. Field of the Invention
This invention pertains to pitch control systems for helicopter rotors that produce aerodynamic thrust of variable magnitude in accordance with the angular relation of rotor blades to an airstream in which the rotor turns. The invention relates particularly to increasing the service life of rod end bearings that operate to transmit pitch control force to the pitch shafts supported on a rotor hub.
2. Description of the Prior Art
Helicopter rotor hubs are connected driveably to a rotor shaft and the hub-rotor shaft assembly is mounted for rotation about the axis of the rotor shaft. The rotor hub includes several rotor arms, one supporting each rotor blade, each rotor arm angularly spaced about the rotor axis and extending radially along a pitch axis from the rotor axis. Each rotor arm supports a pitch shaft concentrically with the associated pitch axis, about which the corresponding blade moves in accordance with pitch control forces transmitted from a control system.
Each pitch shaft is connected, at a position offset from the pitch axis, to a pitch link or pitch control rod attached to a rotating, axially moveable drive scissors, which moves along the rotor axis in response to actuation by the pitch control system. Connection of the pitch links to the drive scissors and pitch shafts is made by universal joints in the form of rod end bearings carried at opposite ends of each pitch link.
Each pitch arm and pitch shaft is inclined with respect to a plane perpendicular to the rotor shaft; therefore, the end of each pitch link connected to the drive scissors moves along the rotor axis but the other end moves about the inclined pitch axis.
The universal joints accommodate the motion at each end of the pitch links about different axes as the pitch links transmit force from the control system to the pitch shafts by rotating on a spherical surface integral with each rod end bearing.
Centrifugal forces, developed by rotation of the rotor, continually produce contact forces among the components of the rod end bearings. These forces operate to reduce the service life of the rod end bearings on the pitch links because they increase the magnitude of forces on the bearings during operation as pitch control force is transmitted from the drive scissors to the pitch shafts.